The present invention relates in general to the fabrication of semiconductor materials, more particularly semiconductor hetero-substrates for use in microelectronics, optoelectronics, optics, or photonics.
More precisely, the invention relates to a novel method of fabricating a semiconductor hetero-substrate composed at least of a support and one or more thin layer(s), in which the materials employed and their thermal properties may differ.
Methods of this type are already known in general. For example, it is known to produce hetero-substrates employing bonding techniques, in particular molecular bonding. Non-limiting examples include the methods known as the BESOI®, ELTRAN® and SMART-CUT® methods, all of which employ a bonding step. It will be recalled in this regard that in the context of fabricating hetero-substrates, these methods comprise at least the following steps:
a) bonding by putting into contact two generally-massive substrates made of materials that are in general dissimilar, with a useful layer that rests on a support substrate, the whole forming a hetero-structure;
b) strengthening the bonding interface of the two substrates by carrying out a high temperature heat treatment to reduce the fragility of the interface, thereby avoiding or at least limiting problems of delamination or degradation of the mechanical and/or electrical properties of the useful layer; and
c) reducing the thickness of the useful layer resting on the support substrate to constitute a thin layer on such substrate.
Such steps may be carried out with different variations, such as sacrificial oxidation in step c), or in a different order, in particular by interchanging steps b) and c).
Further, certain of those steps may be combined to optimize the overall method (i.e., the cumulative duration of treatments, cumulative duration associated with manipulation, etc). As an example, a stabilization heat treatment of the bonding interface (step b)) may advantageously be combined with a thinning step (step c)) (see U.S. Pat. No. 6,403,450).
In the context of the production of hetero-substrates with materials having different properties, for example different coefficients of thermal expansion, the heat treatments carried out during fabrication of a composite substrate for strengthening of the interface, thinning, or the like, typically induce large mechanical stresses. Such stresses may result in weakening, followed in some cases by cracking or even fracture of one or both of the treated substrates. Such stresses may also result in irreparable plastic deformation of the treated substrate or substrates. In particular, dislocations and/or slip planes and/or other crystalline defects may appear.
It is also known that the temperatures at which such problems appear typically depend on:                the mechanical energy stored by the composite structure during the heat treatment which is carried out;        the difference between the coefficients of thermal expansion of the component materials of the composite structure; and        the thicknesses of the substrates employed.        
In the context of fabricating hetero-substrates by a SMART-CUT® method, such problems may constitute severe limitations. More particularly, the maximum possible temperatures are reduced, so that heat treatments become difficult to use because of lack of efficiency.
As a non-limiting example, a strengthening heat treatment of an interface at about 1050° C.-1000° C. will be difficult to carry out in the case of a hetero-structure having a useful layer that is 500 Å because thick, the temperatures generally employed in that type of treatment being too high as regards the problems mentioned above.
Further, solutions are known which may improve the reinforcement of the bonding interface of a hetero-substrate without supplying too much energy. A first proposal, known as “plasma bonding”, consists in applying certain treatments to the surfaces to be bonded, to increase the bonding energy for a given strengthening heat treatment. In that way, thermal stresses to which the substrates are subjected are released, while maintaining sufficient strengthening and encapsulating of the interface in the composite structure. However, that proposition requires specific equipment, limiting its economic attraction.
A second known solution consists in carrying out eutectic bonding: a metallic layer (Au2Si3) is interposed between the two substrates to be bonded to facilitate their bonding by heat treatment, so that the temperatures can remain relatively low. Thus, that solution also has the advantage of being able to release thermal stresses in a treatment for strengthening the interface in a hetero-substrate. However, the presence of the metallic layer at the interface limits the maximum temperatures permitted during subsequent steps in the fabrication method; temperatures which are too high could lead to melting of that layer. Further, an additional step is needed for interposing the metallic layer.
Thus, there is a need for better solutions to these problems, and these are now provided by the present invention.